This invention relates to the fabrication of printed circuit boards, and more specifically to a method for drilling small holes in printed circuit boards employing an improved entry overlay sheet.
In the process of manufacturing printed circuit boards used in the electronics industry, many holes of various sizes have to be drilled through these boards to accommodate component leads or to be used as interconnections between different layers of the printed circuit board. Currently, the existing process of drilling holes in printed circuit boards utilizes an entry material sheet of either aluminum, e.g., as described in U.S. Pat. No. 4,019,826, or a laminate sheet having a core of cellulose and/or unwoven glass fiber laminated on both sides with aluminum as the entry material. For example, U.S. Pat. No. 4,311,419 describes an entry material comprising a wood pulp-glass composite substrate having aluminum foil bonded to the opposite surfaces thereof.
In the fabrication of a new generation of printed circuit boards requiring very small holes of diameter 0.005 inches and smaller, the drilling of such very small holes is now a major difficulty. Specifically, due to the very small hole diameter and consequently the very small drill diameter used to drill those holes, the entry material becomes a very critical item enabling successful drilling operations. Due to the fragile nature of the small drill, where the drill diameter is less than 0.005 inches, conventional entry materials cannot be used effectively to guide the drill accurately in the drilled substrate.
Conventional entry materials have several disadvantages. Aluminum is not an easily drilled medium and small drills will frequently break upon contact. The drill is made of tungsten carbide and is very brittle; hence, bending of the drill in the process of penetrating the entry material may result in breakage of the drill. The drilled hole positioning is of great importance in the fabrication of many printed circuit boards, and any irregularities in the entry material may cause deflection of the fine drill away from the true direction, therefore decreasing the drilled hole positioning accuracy. The mixture of cellulose or unwoven fiberglass with various resins in some conventional entry materials is not homogenous when viewed under a microscope. This inhomogeneity tends to deflect the small diameter drills. This deflection can be sufficient to affect the positioning of the drilled hole and often break the drill.